Field of the Invention
The present invention relates to a variable magnification optical system usable in a video camera, an electronic still camera or the like, and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system that is appropriate especially for a surveillance camera and usable in a wide wavelength band of from a visible range to a near-infrared range, and an imaging apparatus including the variable magnification optical system.
Description of the Related Art
Conventionally, surveillance cameras were used for prevention of crimes, recording and the like. An optical system for such a surveillance camera needs to be structurable in small size and at low cost, and to have a large relative aperture to identify a subject even in low illumination imaging conditions and also high optical performance.
Further, surveillance cameras for both day and night are installed often in unattended facilities, and imaging in visible light is performed during day time, and imaging in near-infrared light is performed at night. Therefore, the optical system for such a surveillance camera needs to maintain high optical performance by excellent correction of chromatic aberrations in a wide wavelength band of from a visible range to a near-infrared range.
As a variable magnification optical system in which chromatic aberrations are excellently corrected from a visible range to a near-infrared range, for example, an optical system disclosed in International Patent Publication No. 2012/153505 (Patent Document 1) is known. The optical system disclosed in Patent Document 1 includes a negative first lens group and a positive second lens group in this order from the object side. The optical system is configured in such a manner that magnification is changed by changing a distance between the two lens groups and correction of the position of an image plane necessitated by the change in magnification is performed by moving the first lens group.